I know this part of the story was allegedly scrapped, but i say we are going to find out she was Osiris' assistant
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It is unknown whether or not the Exo Stranger is working with Osiris. Also, yes, the Stranger is a time traveler, she hops around from timeline to timeline, trying to find one where the Darkness doesn't prevail. This is explained in Ghost Fragment: The Exo Stranger 2